1. Field
The present invention relates to a low noise amplifier used in a radio receiver and the like.
2. Description of the Related Art
A radio receiver requires an amplifier that amplifies a small input signal. However, the noise in amplifying an input signal becomes relatively large as the input signal becomes smaller, and the signal to noise ratio (S/N ratio) will decrease. Then, in order to reduce the noise caused by amplification, an LNA (Low Noise Amplifier) has been proposed.
Meanwhile, if the frequency band of an input signal is wide, an impedance of an antenna and an input impedance of a low noise amplifier need to be matched to each other over a wide bandwidth. For this reason, a low noise amplifier configured using an FET (Field Effect Transistor) is usually of a common-gate type, for example Japanese Laid-open Patent Publication No. H05-235656.
Moreover, in cases where the input level of an input signal fluctuates, a function to make the gain variable is required in order to keep the output level of the amplifier constant. For this reason, an amplifier having an attenuator circuit, for example a resistor network, a ladder network, provided in the signal path at a preceding stage of the amplifier has been proposed, for example, in Japanese Laid-open Patent Publication No. H07-249951.
However, in the variable gain amplifier using the attenuator circuit, thermal noise arises from resistors and therefore the noise characteristics will be degraded. In particular, it is undesirable to provide an attenuator circuit at a preceding stage of the low noise amplifier.